1. Field of the Invention
Sodium borohydride (NaBH.sub.4) is a commercial product used as a source of hydrogen, as a reducing agent and as a blowing agent for plastics. It has been produced from the reaction of sodium hydride and trimethyl borate, as described by Schlesinger in U.S. Pat. No. 2,534,533. This reaction can result in an aqueous solution from which it is necessary to first extract the sodium borohydride with a solvent, such as isopropylamine, and then separate the borohydride product from the extraction solvent. As described in U.S. Pat. No. 3,795,491 to Winiarczyk, the borohydride product can be separated from the solvent and recovered as crystals of sodium borohydride dihydrate (NaBH.sub.4.2H.sub.2 O). The sodium borohydride dihydrate is then dried to remove the water of hydration and form pure solid sodium borohydride. This invention is directed to a new and improved method of drying the sodium borohydride dihydrate to remove the water of hydration therefrom and the unique sodium borohydride product thereby produced.
2. Description of Related Art
Previously, drying of the sodium borohydride dihydrate has been accomplished by controlled heating of the particles, or crystals, under vacuum. The wet particles are loaded as a batch in rotary vacuum dryers having double helical coil ribbon agitators. The dryers are evacuated to a relatively high vacuum and the particles heated by heat transfer through the agitator shaft and dryer jacket. Once the moisture content of the charge is within the desired specification, the borohydride is cooled by extracting heat through the heat transfer surfaces, and the cooled product is then removed from the dryer.
The dried borohydride is a dusty, fine powder which is very hygroscopic and prone to agglomeration. Upon its removal from the dryer, the immediate addition of an anti-caking or flow agent, typically 0.5% by weight of silica or magnesium carbonate, or the immediate mechanical compaction to a compacted product form, is necessary to prevent severe agglomeration. The need to include such additives reduces the purity of the product. Both the reduced product purity and the compacted product form can detrimentally reduce the product's solubility and reactivity.
The prior process is hampered by the difficulty of maintaining a large leak-tight vacuum system with its associated large vacuum sources, and the possibility of massive air intrusion resulting in unsatisfactory product quality. Additional product quality deficiencies result from the inconsistencies typically encountered in batch processing. The agglomerating tendency and dusty nature of the product requires special handling and/or additives which impact on the manufacturing costs and can impact on the product's desired chemical activity. Moreover, since the affiliated processing steps upstream and downstream of the subject drying step are operated as continuous processes, the operation of the drying step as a batch process requires scheduling and storage support which would not be required if it could be operated as a continuous process.
There is a need for a drying technique which alleviates the above noted problems. It is an objective of the inventive process to provide a drying technique which avoids the maintenance and mechanical requirements of a large capacity vacuum system. A further objective is the provision of a drying technique which is capable of continuous operation.
Further objectives of the present invention include providing a dried sodium borohydride product which is not as dusty, or fine, as the vacuum-dried product; and providing a dried product which is free flowing without the need for either an anti-caking/flow agent additive or for immediate compacting.